The present invention relates to an echo canceller for eliminating undesirable echoes occuring in a long-distance telephone network and, more particularly, to an echo canceller suited for a long-distance digitized telephone network where the incoming and outgoing digital signals have different sampling rates.
It is well known that a hybrid circuit connecting a two-wire circuit to a long-distance four-wire circuit in such a telephone network cannot provide echo-free coupling between the transmission and reception paths of the four-wire circuit. The reason for this is that a part of the incoming signal supplied to the reception path of the four-wire circuit leaks to the transmission-side path through the hybrid circuit, causing so-called "talker's" echo at the other end of the long-distance four-wire circuit.
An example of such an echo canceller is shown in the U.S. Pat. No. 4,064,379. The echo canceller disclosed therein is based on total digitization of the signal to be processed, wherein a replica of the echo signal used for cancellation of the true echo.
Generally, in such a long-distance telephone network, the master clock sources disposed at both ends of the four-wire circuit are subject to frequency fluctuation, with the result that the sampling times of the sampling pulses produced by counting-down master clock pulses differ from one another. For this reason, the prior art echo canceller, which suffers from this frequency fluctuation problem, cannot achieve accurate echo cancellation between the talker's echo and an estimated echo.
An object of the present invention is, therefore, to provide an echo canceller free from the disadvantages in the prior art.